I think what you really wanted to ask is which is grammatical of the two sentences: "were you seen by him, he would be surprised", and "were you to be seen by him, he would be surprised". My feeling is that they're both grammatical, that both sound old-fashioned and would probably be worded differently nowadays, but that the first construction is more outdated than the second.
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Peter Shor Jun 20 '12 at 18:21

4 Answers
4

The second is obviously wrong, since it has an extra "be" in it which shouldn't be there. But the first also uses the wrong tense.

Let's analyze this by untangling the passive (which makes it very confusing). First, removing the inversion, you get "you were seen by him". The active voice construction corresponding to "you were seen by him" is
"he saw you". So, taking the first clause out of the passive, you get

*If he saw you, he would have been surprised.

But these tenses don't agree. It should be

If he had seen you, he would have been surprised.

Putting this back in the passive, and putting back the inversion, the correct phrasing is: "Had you been seen by him, he would have been surprised."